(1) Field of the Invention
The invention relates to a method for removing sulfur dioxide from a flue gas stream and more particularly relates to a method for removing oxides of sulfur from a waste gas stream in a wet scrubber using fly ash as an additive with other sulfur dioxide reactant materials.
(2) Description of the Prior Art
In the combustion of fossil fuels, particularly in steam boilers of electric power plants, fly ash and oxides of sulfur as well as other air polluting materials are by-products of said combustion. In recent years it has become common place to utilize large quantities of coal having large percentages of sulfur therein as a source of combustible material in said electric power plants. In these plants, many different types of systems have been devised for removing the fly ash materials as well as the oxides of sulfur. The most common system utilized is one that includes an electrostatic precipitator and a wet scrubbing tower, the electrostatic precipitator being utilized to remove the fly ash with a wet scrubber being utilized to remove the oxides of sulfur. Generally, the electrostatic precipitators are placed upstream of the wet scrubbing tower and the fly ash is removed first, as well as the other particulate materials in the gas stream, with only oxides of sulfur and other non-particulates remaining in the gas stream. In the removal of the oxides of sulfur from the flue gas stream, the flue gas stream is passed through a wet scrubber in which the scrubbing solution generally includes additives which will chemically combine with the oxides of sulfur to form precipitates which may then be easily removed from the gas stream in slurry form. The additive which is generally used is a calcium containing material such as that obtained from lime or limestone.
As we use more coal in the generation of electric power, it has become common practice to use coal which includes a significant amount of alkali. In this high alkali coal, the coal contains from about 0.3 to 1.5 percent by weight of sulfur and between about 3 and 25 percent by weight of fly ash, the fly ash content having a total calcium, sodium, and magnesium content in the range of from about 8 to 50 percent by weight of the fly ash. However, even though the fly ash is high in alkali, up until the present invention means for utilizing the alkali in the fly ash in an automatically controlled process system had not been found successful.